There are distinct kinds of DNA damage caused by environmental chemicals and ionizing radiation (IR). However, double strand breaks (DSBs) in DNA are among the most serious. Failure to repair or gross misrepair of DSBs results in cell death or sometimes, in the case of humans, somatic cell diseases such as cancer. Knockout mice and work with cultured cells has demonstrated that DSB repair in vertebrates is much more complex than previously imagined. It was thought that most DSB repair in mammals was by non-homologous end-joining (NHEJ), but it is now clear that homologous recombination (HR) and other homology-directed processes play key roles in maintaining genome integrity. Further, these mechanistically distinct processes interact in novel ways with each other as well as those for mismatch repair (MMR) and apoptosis. We developed a mouse model that detects somatic genetic endpoints of DSB and other kinds of repair. These include mitotic recombination (MR), interstitial deletion and chromosomal deletion and translocation, all of which may result in loss of heterozygosity (LOH). This model also allows for the quantitation and characterization of point mutation, chromosome loss and epigenetic gene inactivation leading to LOH - key biological events to the etiology of cancer and diseases such as polycystic kidney disease. By introducing knockout defects of DNA repair or apoptosis into our model and by exposing these mice to IR or DNA cross-linking agents such as mitomycin C and cisplatin we will begin to understand the roles and regulation of competing DSB repair processes. Specific aims include: 1. To characterize the LOH frequency and spectrum of DAP-resistant (DAPR) skin fibroblasts and T cells in Aprtneo/+ mice after in utero or adult exposure to ionizing radiation (IR). 2. To investigate the effect of mitomycin C on LOH in Aprtneo/+ mice. 3. To introduce deficiency for each of the MMR gene products and note effects on the MR stringency requirement for DNA sequence homology. 4. To investigate whether DSB and MMR repair pathways both participate in repair of damage induced by mitomycin C or cisplatin. 5. To introduce Ku70 deficiency to test whether or not NHEJ and HR have overlapping functions. 6. To introduce DNA-PKcs deficiency to test whether or not Ku and DNA-PKcs participate in distinct as well as common DSB repair pathways.